Generators of the claw pole type, having passive bridge rectifiers, are conventionally utilized in passenger cars. The output of such generators is adjusted via the excitation field, and that in turn via the excitation current. The output voltage furnished by the generator via the bridge rectifier can be held constant, regardless of network load, rotation speed, and temperature, by regulating the excitation field.
When what is discussed hereinafter is simply a “generator,” this can refer to an electric machine operable in both generator mode and motor mode, for example a so-called “starter generator.” The present invention is suitable not only for claw pole-type generators but instead for all electric machines operable at least in generator mode. In passenger cars, bridge rectifiers in a six-, eight-, or ten-pulse configuration, corresponding to the three-, four-, or five-phase generators that are usually installed, are usually used. The present invention is also suitable, however, for bridge rectifiers having different numbers of phases.
A load discontinuity in the connected network, for example due to connection or disconnection of a load, results in a load discontinuity at the generator. But because the power delivery of the generator cannot be modified arbitrarily quickly due to the inductance of the excitation field, the generator current initially remains constant, which in the context of a load dump can result in an appreciable increase in the output voltage. Dissipation of the excitation field can take several hundred milliseconds.
As long as a battery is present in the vehicle electrical system, that battery generally can absorb the excess generator output and thus prevent an excessive voltage rise. If a battery is not present, however, the output voltage then rises very quickly and is capable of damaging electrical system components and/or the generator.
In generators having passive bridge rectifiers, this is prevented by using Zener diodes as rectifier diodes. The Zener diodes clamp the output voltage above their breakdown voltage, and are therefore capable of absorbing excess current and converting it into heat. Reliable operation of the generator is thereby ensured.
Controllable current valves capable of being switched on and shut off, in particular MOSFETs, can also be used instead of diodes in bridge rectifiers; corresponding bridge rectifiers are then referred to as “active” bridge rectifiers. An advantage is their lower power loss in the switched-on state, and thus better efficiency especially at part load.
The current valves can be controlled in centralized or decentralized fashion. A “centralized” control system is understood to mean that one common control unit monitors all the alternating current phases and controls all the current valves, and optionally also the excitation field of the generator. A “decentralized” control system is understood to mean that one control unit respectively controls one generator phase, and controls, as a function of the phase voltage, only the current valves associated with the respective phase, i.e. only the current valves of a respective half-bridge of the bridge rectifier. Typically, no communication takes place between individual decentralized control units.
In the context of active bridge rectifiers, one possibility for preventing voltage spikes in the vehicle electrical system in the event of a load dump is to switch on the respective current valves of the upper or the lower rectifier branch (i.e., all the high-side or all the low-side current valves) in all the half-bridges. The result is that the electric machine is internally short-circuited but not the connected network, since the current valves of the respective other rectifier branch are not switched on.
The measures just explained are also referred to hereinafter as a “phase short circuit.” According to the terminology used here, a phase short circuit is therefore initiated by switching on (making conductive) all the current valves of the respective rectifier branch, and correspondingly discontinued by shutting off those current valves. The semiconductor valves are switched on in this context by furnishing a corresponding control voltage to their gate terminal (addressing), with the result that the drain-source section of the semiconductor valves becomes conductive or low-impedance. The semiconductor valves are correspondingly shut off by terminating the provision of control voltage, and the drain-source section thus becomes non-conductive or high-impedance. In the absence of a phase short circuit, ordinary rectifier operation prevails.
A phase short circuit can be initiated, for example, when the voltage between the DC voltage terminals of the bridge rectifier (usually referred to as B+ and B−), or between the voltage-carrying DC voltage terminal and ground, exceeds an upper threshold value. The phase short circuit can be discontinued again when that voltage then falls below a lower threshold value. Time-based control can also be used.
If the excitation field is not yet sufficiently dissipated at the point in time at which the phase short circuit is discontinued, the voltage between the DC voltage terminals of the bridge rectifier rises again after the phase short circuit is discontinued, and again exceeds the upper threshold value. Phase short circuits are therefore repeatedly initiated and discontinued until the excitation field is completely or sufficiently dissipated. As mentioned, the excitation field can take several hundred milliseconds to dissipate, while the switching phases of initiation and discontinuation of the phase short circuits are typically only a few milliseconds long. Until the excitation field dissipates there is therefore a back-and-forth switchover between phase short circuits and ordinary rectification over a considerable time span, which is also referred to hereinafter as the “de-excitation” time span. Considerable power losses occur in this context, in particular during the phase short circuits. This can result in appreciable stress on and premature failure of the participating current valves.
It is therefore desirable to reduce the stress on corresponding current valves during the de-excitation time span.